Compression ignition diesel engines have great utility and advantage as vehicle power trains because of their inherent fuel economy and high torque at low speed. Diesel engines run at a high air to fuel (A/F) ratio under very fuel lean conditions. Because of this, they have very low emissions of gas phase hydrocarbons and carbon monoxide. However, diesel exhaust is characterized by relatively high emissions of nitrogen oxides (NOx) and particulates. Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Emissions treatment systems for diesel engines must treat all of the components of the exhaust to meet emissions standards set by various regulatory agencies throughout the world.
The total particulate matter emissions of diesel exhaust contain three main components. One component is the solid, dry, carbonaceous fraction or soot fraction. This dry carbonaceous fraction contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The SOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel. The third component of the particulate matter is the so-called sulfate fraction, which is formed from small quantities of sulfur components present in the diesel fuel.
Catalyst compositions and substrates on which the compositions are disposed are typically provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For instance, oxidation catalysts, which, in diesel engine systems may be referred to as diesel oxidation catalysts (DOCs), containing platinum group metals, base metals and combinations thereof, facilitate the treatment of diesel engine exhaust by promoting the conversion of both unburned hydrocarbons (HC) and carbon monoxide (CO) gaseous pollutants, and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been disposed on various substrates (e.g., honeycomb flow through monolith substrates), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. Certain oxidation catalysts also promote the oxidation of NO to NO2.
In addition to the use of oxidation catalysts, diesel particulate filters are used to achieve high particulate matter reduction in diesel emissions treatment systems. Known filter structures that remove particulate matter from diesel exhaust include honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust.
Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite or silicon-carbide. Wall flow substrates are particularly useful to filter particulate matter from diesel engine exhaust gases. A common construction is a multi-passage honeycomb structure having the ends of alternate passages on the inlet and outlet sides of the honeycomb structure plugged. This construction results in a checkerboard-type pattern on either end. Passages plugged on the inlet axial end are open on the outlet axial end. This permits the exhaust gas with the entrained particulate matter to enter the open inlet passages, flow through the porous internal walls and exit through the channels having open outlet axial ends. The particulate matter is thereby filtered on to the internal walls of the substrate. The gas pressure forces the exhaust gas through the porous structural walls into the channels closed at the upstream axial end and open at the downstream axial end. The accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure.
Catalyst compositions deposited along the internal walls of the wall flow substrate assist in the regeneration of the filter substrates by promoting the combustion of the accumulated particulate matter. The combustion of the accumulated particulate matter restores acceptable back pressures within the exhaust system. Soot combustion can be passive (e.g., with catalyst on the wall flow filter and adequately high exhaust temperatures), though for many applications active soot combustion is also required (e.g., production of a high temperature exotherm in the exhaust up-stream of the filter). Both processes utilize an oxidant such as O2 or NO2 to combust the particulate matter.
Passive regeneration processes combust the particulate matter at temperatures within the normal operating range of the diesel exhaust system. Preferably, the oxidant used in the regeneration process is NO2 since the soot fraction combusts at much lower temperatures than those needed when O2 serves as the oxidant. While O2 is readily available from the atmosphere, NO2 can be generated through the use of upstream oxidation catalysts to oxidize NO in the exhaust stream. An example of a passive regeneration process is disclosed in U.S. Pat. Nos. 6,753,294 and 7,097,817
Active regeneration processes are generally needed to clear out the accumulated particulate matter, and restore acceptable back pressures within the filter. The soot fraction of the particulate matter generally requires temperatures in excess of 500° C. to burn under oxygen rich (lean) conditions, which are higher temperatures than those typically present in diesel exhaust. Active regeneration processes are normally initiated by altering the engine management to raise temperatures in front of the filter up to 500-630° C. Depending on driving mode, high exotherms can occur inside the filter when the cooling during regeneration is not sufficient (low speed/low load or idle driving mode). Such exotherms may exceed 800° C. or more within the filter. One common way that has been developed to accomplish active regeneration is the introduction of a combustible material (e.g., diesel fuel) into the exhaust and burning it across a flow-thru diesel oxidation catalyst (DOC) mounted up-stream of the filter. The exotherm from this auxiliary combustion provides the sensible heat (e.g. about 500-700° C.) needed to burn soot from the filter in a short period of time (e.g. about 2-20 min.).
An example of a system is shown in U.S. Pat. No. 6,928,806. The DOC functions during active regeneration mode to light-off and burn fuel injected into the low temperature (e.g., about 250-300° C.) exhaust (directly or via the engine) and thereby produce an exotherm to heat the exhaust entering the particulate filter to the temperatures required (about 500-650° C.) to combust accumulated soot from the filter, thereby regenerating the filter to reduce the operating pressure drop across the filter associated with the soot accumulation.
High material costs associated with platinum group metal-containing compositions augment the need to slow or prevent the degradation of catalyst coatings due to active regeneration events. Catalyst coatings disposed on wall flow filters often contain platinum group metal components as active catalyst components to ensure acceptable conversions of the gaseous emissions (HC, CO) of the diesel exhaust to innocuous components (e.g., CO2, H2O). The loadings of such components are generally adjusted so that the catalyst substrate meets emissions regulations even after catalyst aging. Consequently, coating designs that maximize the efficiency and durability of platinum group metal usage along the substrate are desirable.
Certain conventional coating designs for wall flow substrates have a homogeneous distribution of catalyst coating along the entire axial length of the internal walls. In such designs the platinum group metal concentrations are typically adjusted to meet the emissions requirements under the most stringent conditions. Most often such conditions refer to the catalyst's performance after the catalyst has aged. The cost associated with the required platinum group metal concentration is often higher than is desired.
As can be appreciated from the above, current particulate filter systems pose a number of issues concerning precious metals material costs and degradation of the catalyst on the particulate filter due to exposure to high temperatures. Accordingly, it would be desirable to provide alternatives diesel engine in exhaust treatment systems and methods that alleviate one or more of these issues.